Cetirizine, chemically [2-[4-[(4-chlorophenyl)phenylmethyl]-1-piperazinyl]ethoxy]acetic acid is an antihistamine non-sedating type histamine H1-receptor antagonist, indicated for relief of symptoms associated with seasonal allergic rhinitis, perennial allergic rhinitis and related diseases.

U.S. Pat. No. 4,525,358 and its equivalent EP 58146 disclose cetirizine and its pharmaceutically acceptable salts. The process for the synthesis of cetirizine comprises condensation of 1-[(4-chlorophenyl)-phenylmethyl]piperazine with 2-chloroethoxy acetamide to obtain 2-[2-[4-[(4-chlorophenyl)phenylmethyl]-1-piperazinyl]-ethoxy acetamide which on hydrolysis gives cetirizine.
It was found later that the pharmacological activity resides primarily in (R)-isomer or (−) form known as levocetirizine. GB 2225321 describes a process for the preparation of the dextro and levorotatory isomers of cetirizine comprising hydrolysis of 2-[2-[4-[(4-chlorophenyl)phenylmethyl]-1-piperazynyl]-ethoxy acetonitrile.

(−)-1-[(4-chlorophenyl)-phenylmethyl]piperazine is a very important intermediate in the synthesis of levocetirizine. U.S. Pat. No. 5,478,941 discloses a process for the synthesis of (−)-1-[(4-chlorophenyl)-phenylmethyl]piperazine involving hydrolyzing 1-[(4-chlorophenyl)-phenylmethyl]-4-(4-methylphenyl)sulfonyl piperazine with hydrobromic acid in the presence of 4-hydroxybenzoic acid.

The alternative routes of synthesis of (−)-1-[(4-chlorophenyl)-phenylmethyl]piperazine disclosed in the prior art involve the use of bis chloro ethylamine which is carcinogenic in nature.
Levocetirizine is a highly-potent non-sedating anti-allergic agent. Hence, there are continuous attempts to develop new processes for the synthesis of levocetirizine and its intermediates. The present invention describes a new process for the preparation of the key intermediate (−)-1-[(4-chlorophenyl)-phenylmethyl]piperazine.